An image sensor such as a CCD sensor or CMOS sensor sometimes generates streak-like fixed pattern noise called a “vertical streak” in a captured image owing to nonuniformity of a dark current in a vertical transfer path and degradation in transferring. As a method for correcting the generated fixed pattern noise, a technique, where the output signal (black image) of an image sensor that is obtained in a light-shielded state is stored in advance, and subtracted from a captured image (black subtraction process), is known.
However, the black image contains not only fixed pattern noise but also random noise. In an image obtained by subtracting a black image from a captured image, random noise stands out much more than in an image before subtraction. To prevent this, a technique is known whereby random noise from a black image is reduced to generate an extraction image which extracts streak-like fixed pattern noise.
Japanese Patent Laid-Open No. 2007-028026 discloses a technique of reducing random noise by applying a median filter using a row or column of an image sensor as one process unit. Japanese Patent Laid-Open No. 2006-311086 discloses a technique of storing a plurality of black images and suppressing uncorrelated random noise contained in these black images.
However, streak-like fixed pattern noise may change in streak intensity for each region even on a single streak. For this reason, applying a random noise reduction filter using a row or column as a unit may generate a streak in a region low in streak intensity or a region free from a streak owing to overcorrection like Japanese Patent Laid-Open No. 2007-028026. Also, obtaining a plurality of black images, like Japanese Patent Laid-Open No. 2006-311086, may increase an area for storing these black images and prolong the process time by a plurality of black image capturing processes.